In the typical nuclear magnetic resonance (NMR) analysis instrument, a sample is placed within a volume situated within a homogeneous region of magnetic field. Excitation and detection of resonance is obtained from a suitably placed coil (or coils) ordinarily closely spaced with respect to the sample and preferably enveloping it. Quite typically for modern FI resonance spectroscopy, the sample is contained within a cylindrical tube coaxial with, and within a single coil and means are provided to rapidly rotate the sample tube about its axis to average any residual inhomogeneities of the magnetic field.
The material environment of the sample volume of typical prior art apparatus may contain a number of substances: the sample container, usually glass anid possibly including a stopper delimiter of nylon or similar inert material; a conductive material forming the RF coil conductor, commonly copper, aluminum, silver or gold; a coil form supporting the coil; a bonding agent for securing the conductor to the coil form; one or more holes in the coil form for necessary innerconnection of coil winding components; and, air permeating all available spaces. These materials, distinct from the sample and solvent itself, exhibit various magnetic susceptibilities and contribute influence to the signal. The relative rotation of the sample and the RF field acts to average sources of magnetic perturbations exhibiting non-cylindrical symmetry. The perturbation is effectively smeared over in imaginary cylindrically symmetric volume as a consequence of the rotation.
Some sources of magnetic perturbation have been considered in prior art compensatory schemes. Coil materials and the bonding agent materials have been considered by Anderson, et al., U.S. Pat. No. 3,091,732 where it was sought to provide coil materials and bonding agents for securing the coil to a coil form, which materials were required to exhibit a magnetic susceptibility approximating air (in which these components are necessarily submerged). The inhomogeneity due to the holes in the coil form is treated in copending U.S. Ser. No. 483,351, now U.S. Pat. No. 4,517,526 commonly assigned with the present invention.
The rapid rotation or spinning of the sample, while effective in averaging perturbations by rotation about an axis, cannot remove inhomogeneities in axial distribution of the sample. One such inhomogeneity has been observed in U.S. Pat. No. 4,077,002 to be concommitant with the spinning process itself in producing a curved miniscus due to the centripetal forces acting on a fluid sample. A simple expedient of the above patent took the form of a plug of polyfluorocarbon or like material inserted in the sample holder to float partially submerged in the sample and to constrain the sample more nearly to the desired volume. It is apparent, however, that the unspecified magnetic susceptibility of the polyfluorocarbon plug introduces a magnetic perturbation.
It has also been recognized that ordinary edge effects are to be avoided wherein the sample does not fill the sample volume and an empty portion of the volume thereby constitutes a region of distinct magnetic susceptibility. It is common practice to utilize very small bore tubes of length greater than the axial length of the sample volume, filled with sample well beyond the sensitive region. When availability of sample material is limited, however, this practice necessitates increased dilution and a consequent reduction in signal intensity.
Accordingly, it is an object of the present invention to enhance the sensivity of NMR analysis of samples by homogenizing the magnetic environment of the sample volume.
In one feature of the invention, the sensitive volume occupied by the sample is delimited by inert plugs which exhibit magnetic susceptibility substantially equal to that of the solvent in which the sample is presented for analysis and in particular that region of the sensitive volume which supports the best RF homogeneity is selected to contain the sample for RF irradiation.
In another feature of the invention, the inert plugs are so constituted to present a desired magnetic susceptibility tailored to be substantially equal to the solvent in which the sample is presented for analysis.
In still another feature of the invention, said inert plugs are so constituted to minimize that portion of signal intensity degenerate with the signal intensity characteristic of the sample.
In again another feature of the invention, the solvent is so constituted as to exhibit a characteristic volume magnetic susceptibility substantially equal to the inert plugs utilized to delimit the sample volume.
The principal nature and utility of this invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.